Automotive vehicles with an internal combustion engine have an exhaust system including a pathway for exhaust gas to move away from the engine. Depending on the desired operating state, internal combustion engines can be operated with fuel/air ratios in which (1) the fuel constituent is present in a stoichiometric surplus (rich range), (2) the oxygen of the air constituent is stoichiometrically predominant (lean range), and (3) the fuel and air constituents satisfy stoichiometric requirements. The composition of the fuel-air mixture determines the composition of the exhaust gas. In the rich range, considerable quantities of nonburned or partially burned fuel are found, while the oxygen has been substantially consumed and has nearly disappeared. In the lean range, the ratios are reversed, and in a stoichiometric composition of the fuel-air mixture, both fuel and oxygen are minimized.
It is well known that the oxygen concentration in the exhaust gas of an engine has a direct relationship to the air-to-fuel ratio of the fuel mixture supplied to the engine. As a result, gas sensors, namely oxygen sensors, are used in automotive internal combustion control systems to provide accurate oxygen concentration measurements of automobile exhaust gases for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
A switch type oxygen sensor, generally, comprises an ionically conductive solid electrolyte material, a sensing electrode that is exposed to the exhaust gas and reference electrode that is exposed to a reference gas, such as air or oxygen, at known partial pressure. It operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force, which can be described by the Nernst equation:       E    =                  (                              R            ⁢                                                   ⁢            T                                4            ⁢            F                          )            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                                              w            ⁢                                                   ⁢            h            ⁢                                                   ⁢            e            ⁢                                                   ⁢            r            ⁢                                                   ⁢                          e              :              E                                =                    ⁢                      electromotive            ⁢                                                   ⁢            force                                                        R          =                    ⁢                      universal            ⁢                                                   ⁢            gas            ⁢                                                   ⁢            constant                                                        F          =                    ⁢                      Faraday            ⁢                                                   ⁢            constant                                                        T          =                    ⁢                      absolute            ⁢                                                   ⁢            temperature            ⁢                                                   ⁢            of            ⁢                                                   ⁢            the            ⁢                                                   ⁢            gas                                                                    P                          O              2                        ref                    =                    ⁢                      oxygen            ⁢                                                   ⁢            partial            ⁢                                                   ⁢            pressure            ⁢                                                   ⁢            of            ⁢                                                   ⁢            the            ⁢                                                   ⁢            reference            ⁢                                                   ⁢            gas                                                                    P                          O              2                                =                    ⁢                      oxygen            ⁢                                                   ⁢            partial            ⁢                                                   ⁢            pressure            ⁢                                                   ⁢            of            ⁢                                                   ⁢            the            ⁢                                                   ⁢            exhaust            ⁢                                                   ⁢            gas                              The large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point; the switch-like behavior of the sensor enables engine combustion control about stoichiometry. Stoichiometric exhaust gas, which contains unburned hydrocarbons, carbon monoxide, and oxides of nitrogen, can be converted very efficiently to water, carbon dioxide, and nitrogen by automotive three-way catalysts in automotive catalytic converters. In addition to their value for emissions control, the sensors also provide improved fuel economy and drivability.
Further control of engine combustion can be obtained using amperometric mode exhaust sensors, where oxygen is electrochemically pumped through an electrochemical cell using an applied voltage. A gas diffusion-limiting barrier creates a current limited output, the level of which is proportional to the oxygen content of the exhaust gas. These sensors typically consist of two or more electrochemical cells; one of these cells operates in potentiometric mode and serves as a reference cell, while another operates in amperometric mode and serves as an oxygen-pumping cell. This type of sensor, known as a wide range or linear air/fuel ratio sensor, provides information beyond whether the exhaust gas is qualitatively rich or lean; it can quantitatively measure the air/fuel ratio of the exhaust gas.
Due to increasing demands for improved fuel utilization and emissions control, more recent emphasis has been on wide range oxygen sensors capable of accurately determining the oxygen partial pressure in exhaust gas for internal combustion engines operating under both fuel-rich and fuel-lean conditions. Such conditions require an oxygen sensor that is capable of rapid response to changes in oxygen partial pressure by several orders of magnitude, while also having sufficient sensitivity to accurately determine the oxygen partial pressure in both the fuel-rich and fuel-lean conditions.
The temperature of the exhaust gases ranges from ambient temperature, when the engine has not been run recently, to higher than 1,000° C. Since air-fuel ratio output signal depends largely on the exhaust gas temperature, temperature compensation is needed. A heater assists an oxygen sensor, in making more precise measurements over a wide range of exhaust gas temperatures, especially when the exhaust gas temperature is low. The addition of the heater also helps to decrease the light-off time of the sensor, that is the time that it takes for the sensor to reach the minimum temperature for proper operation.
Reduction of light-off times has been accomplished through the use of high power heaters. One method for further decreasing light-off times while using only small or modest heating power is to substantially decrease the size of the sensing element, especially the electrolyte. Similarly, during low temperature operation (e.g., about 350° C. or less), the switching time, or time required for the sensor to detect a change from rich to lean or lean to rich exhaust gas compositions, must be as low as possible, preferably below about a half second (500 milliseconds).
The internal resistance of the sensor is further factor that should be controlled. A low internal resistance or impedance will allow the sensor to sink or source more useful current from the monitoring system that is being used for determining the oxygen content of the exhaust gas.